In four cases submitted for our study, we demonstrated the accepted micromorphological criteria for Hairy Cell Leukemia, mononuclear clear cells with numerous cytoplasmic projections at the periphery. Zipper-like junctions were demonstrated in these cells as well as their ability to phagocytose microorganisms. These are properties of cells of the monocytic series, and strongly suggest the non-lymphoid origin of Hairy Cells. Adhesiveness to glass and plastic and responsiveness to isolectins were demonstrated in all cases. Glucose utilization and thymidine-uptake were determined. Cells obtained by filtration leucapheresis using current methods were demonstrated by SEM,TEM, cyto-chemical and functional testing to be altered during harvesting and processing for transfusion. In a model system we demonstrated that washing harvested cells in HBSS and brief incubation in heparinized autologous plasma significantly restored them structurally and functionally.